The present invention relates generally to a tool for shaping curved surfaces on workpieces, and in particular the upper curved surfaces of the fore and aft rails of a gas turbine engine shroud section. The present invention further relates to a method for shaping these curved surfaces with this shaping tool.
The turbines and compressors of gas turbine engines such as jet engines each include one or more circumferentially extending rows or stages of rotating rotor blades which are axially spaced between rows or stages of fixed stator vanes. Each rotor blade has a blade root mounted to the rotor disk, and an airfoil extending radially outwardly from the root which terminates at a blade tip. In many gas turbine engine designs, a number of abutting, circumferentially extending shroud segments or sections are carried by the turbine or compressor case to form an essentially continuous cylindrical-shaped surface along which the tips of the rotor blades tangentially pass. Each of these shroud sections includes an outer face, and an inner, arcuate-shaped face along which the blade tips pass, opposite end portions which abut with adjacent shroud sections and opposed side mounting rails which mount to stationary hangers on the casing of the turbine and/or compressors.
A representative embodiment of one such shroud assembly 10 is disclosed in commonly assigned U.S. Pat. No. 5,165,847 (Proctor et al), issued Nov. 24, 1992. As shown in FIG. 1 of the Procter et al patent, shroud assembly 10 includes a shroud in the form of an annular array of arcuate shroud sections 22 which are held in position by an annular array of arcuate hanger sections 24 supported by the engine outer case 26. Each shroud section 22 is provided with an arcuate or curved base 44 having a radially outwardly extending fore rail 46 and a radially outwardly extending aft rail 48 that are connected by a pair of laterally spaced side rails 50. Shroud section fore rail 46 is provided with a forwardly extending flange 54 which overlaps a flange 56 rearwardly extending from hanger section fore rail 28. An underlying flange 60 rearwardly extending from shroud section aft rail 48 overlaps with hanger flange 58 that extends from hanger section aft rail 30. Flanges 58 and 60 are held in this overlapping relation by an annular C-shaped retaining ring 62. The upper flange 54 of fore rail 46 and upper flange 60 of aft rail 48 that extend between side rails 50 each have a convex arcuate or curved shape.
Shroud sections such as those shown in the Procter et al patent are often made from hard to machine nickel alloys and are typically turned or ground to generate the critical locating surfaces, such as the fore and aft rails. Because these shroud sections are exposed to elevated temperatures in an oxidizing atmosphere, they are often provided with environmental protection in the form of metallic coatings. Methods for applying such metallic coatings include depositing a vapor of one or more protective metals at high temperatures, for example aluminum or alloys of aluminum, to provide, for example, an aluminide coating on the shroud section.
A protective coating, such as an aluminide coating, is typically not required for the curved surfaces of the upper flanges of the fore and aft rails of the shroud section. However, it is usually not economically feasible to selectively mask these surfaces to prevent them from being coated during the coating process. Because of variations that exist in such coating methods, the coating thickness can also differ over the various portions of the shroud section. As a result, the coated shroud section can be become oversized.
In addition, the coating thickness on the curved surfaces of the upper flanges of the fore and aft rails can vary enough to alter the dimensional shape of these rails. Besides variations in coating thickness, the dimensional shape of the upper flanges of the fore and aft rails can become distorted as a result of the heat used in the coating process. While these variations in dimensional shape of the fore and aft rails are usually relatively minute, they can be enough to require reshaping so that the shroud sections can be connected to each other and to their appropriate hanger sections.
Reshaping of these oversized and/or distorted shroud sections typically requires the removal of relatively minute amounts of material (e.g., the coating, the underlying metal or both) from the curved surfaces of the upper flanges of the fore and aft rails. The amount of material required to be removed from these curved surfaces to achieve the desired reshaping is typically on the order of a fraction of a thousandth of an inch, i.e., a fraction of a mil. Conventional machining processes typically cannot remove such small amounts of material effectively. Grinding rework processes to remove such small amounts of material can also be expensive and tedious to set up.
Accordingly, it would be desirable to provide a relatively simple, inexpensive and easy to use tool and method for reshaping the upper curved surfaces of the fore and aft rails of a gas turbine engine shroud section that has become oversized, distorted or otherwise requires reshaping. It would also be desirable to provide a relatively simple, inexpensive and easy to use tool and method for shaping or reshaping the curved surfaces of other workpieces besides gas turbine engine shroud sections to a desired configuration.
The present invention relates to a shaping tool for a workpiece having at least two laterally spaced curved surfaces. The shaping tool comprises:
(a) a base member for securing the workpiece;
(b) a shaping member movable relative to the base member and having one shaping element for each curved surface of the workpiece, each shaping element being laterally spaced and positioned relative to the respective curved surface to permit shaping of that curved surface as the shaping member is moved relative to the base member;
(c) a guide member associated with one of the base member and the shaping member;
(d) a follower member associated with the other of the base member and the shaping member;
(e) the guide member and the follower member cooperating to guide the shaping member through a path as the shaping member is moved relative to the base member such that each of the shaping elements of the shaping member shape the respective curved surfaces of the workpiece.
The present invention also relates to a method for shaping the curved surfaces of the workpiece with this shaping tool. This method comprises the steps of:
(a) securing the workpiece to the base member;
(b) positioning the shaping member relative to the base member so that each shaping element of the shaping member is capable of shaping the respective curved surface of the workpiece;
(c) moving the shaping member in a path such that each of the shaping elements of the shaping member shape the respective curved surfaces of the workpiece; and
(d) repeating step (c) until the desired degree of shaping of the curved surfaces of the workpiece is achieved.
The shaping tool of the present invention and method of using same provides a number of benefits in shaping workpieces that having a plurality of (i.e., at least two) laterally spaced curved surfaces. The shaping tool of the present invention is relatively simple, inexpensive and easy to use in effectively removing relatively small amounts of material (e.g., fractions of a mil) from curved surfaces of workpieces. It can be used to shape curved surfaces of workpieces made from a variety of materials and can be used to shape curved surfaces that lie in either the same or substantially the same curve plane, as well as curved surfaces that lie in different, latitudinally (i.e., vertically) spaced curve planes. The shaping tool and method of the present invention can also allow controlled removal of relatively small amounts of material from the curved surfaces of the workpiece such that the desired degree of shaping of the curved surfaces of the workpiece is automatically, reproducibly and repeatedly achieved.